1. Field of the Invention
The present invention relates to a recording medium transporting mechanism in which a roll paper is used as a recording medium and a sheet of paper is transported from the roll paper. The present invention also relates to an image forming apparatus, such as a printer, a plotter, a facsimile machine, and a copier, including the recording medium transporting mechanism.
2. Description of the Related Art
In electrophotographic image forming apparatuses, such as large-scale copiers and printers, and other printing devices that handle originals of drawings and the like in large sizes, such as A0 size and A1 size, a following mechanism is used when image formation is performed using a  large-size recording medium, to achieve user-friendliness in handling of paper serving as the recording medium. In the mechanism, a roll paper is provided as the recording medium. A sheet of paper is dispensed from the roll paper, and the dispensed sheet of paper is transported towards an image forming section.
Typically, a plurality of roll papers of different paper qualities and paper sizes are mounted on a paper feeding unit of the image forming apparatus. Paper is selected based on a size of an original, and the selected paper is fed to a recording unit. The selected paper is dispensed from the roll paper. A cutter, disposed further upstream from a transfer position of an image carrier, cuts the dispensed paper based on a length of the original. An image is transferred onto the cut paper (sheet of paper). The transferred image is fixed onto the sheet of paper, and image formation is completed. In an actual operation, a reciprocal driving mechanism is provided in a conveying roller device that transports the paper dispensed from the roll paper. After the paper is cut by the cutter, to return a leading end of the roll paper to a standby position (a position further upstream than a paper merging section in which papers from each roll paper merge), the reciprocal driving mechanism is used to operate the conveying roller device in reverse and return the paper. A  control operation such as this is performed to prevent occurrence of a multi-feeding state caused by leading ends of each paper disposed onto a shared conveying path from each roll paper interfering with one another at the paper merging section.
In this manner, the paper is once dispensed from the roll paper and cut to a required length by the cutter. The conveying roller device then operates in reverse and rewinds the leading end of the roll paper to a predetermined position. However, because the roll paper itself does not rotate in a reverse direction, a rewound portion of the paper remains in a bent state or a forcibly folded state between the roll paper itself and the conveying roller device. Deformation, such as a fold, is formed, causing white-out during image formation. Alternatively, wrinkles are formed due to moisture absorption, causing the paper to jam.
Therefore, in Japanese Utility Model Application Laid-open No. H5-85747, a configuration is proposed in which, after the paper dispensed from the roll paper is cut and the paper on the roll paper side is rewound, slack in the paper is prevented by the roll paper itself being rotated in reverse. Specifically, a paper feeding device includes a driving section and a drive transmitting section. The driving section can operate in a forward direction and a  reverse direction, and is connected to a paper feed roller by a one-directional clutch. The drive transmitting section is connected to the driving section by a transmission path other than the one-directional clutch, and can be connected to and released from a gear of a roll paper holder. In the paper feeding device, the drive transmitting section includes a swinging component that is held to allow pivoting towards the gear of the roll paper holder. The swinging component axially supports a main gear that receives rotational torque from the driving section. The main gear meshes with the gear of the roll paper holder as a result of the swinging component pivoting towards the gear of the roll paper holder. The meshing between the main gear and the gear of the roll paper holder is released as a result of the swinging component pivoting in a reverse direction. However, in the proposed configuration, the roll paper rotates in reverse by driving force from a motor. Therefore, a configuration for performing the reverse rotation is required to be added. The proposed configuration is disadvantageous in that the configuration is structurally complex.
In Japanese Patent Application Laid-open No. 2006-248683, to similarly rewind slack in the paper by the roll paper being driven in reverse when the paper on the roll paper side is rewound after the paper dispensed from the  roll paper is cut, a rotation load section and a one-directional transmitting section are provided. The rotation load section is provided on a shaft section of a spool that holds a paper core of the roll paper. The spool can rotate in a transporting direction and a rewinding direction. The one-directional transmitting section is provided within a gear train that transmits drive from a drive source to the rotation load section. The one-directional transmitting section does not transmit drive in the transporting direction of the roll paper and transmits only drive in the rewinding direction.
Conventionally, to hold the roll paper, a spool method or a flange method is used to handle roll papers in various sizes. The spool method used in Japanese Utility Model Application Laid-open No. H5-85747 and in Japanese Patent Application Laid-open No. 2006-248683, is disadvantageous in terms of poor user operability (extreme difficulty in handling) because a long spool is required to be passed through the paper core of the roll paper. However, the spool method is advantageous in that, when a rewinding mechanism and the like are provided, the roll paper itself can be easily driven by the drive transmitting mechanism being provided in the spool. Therefore, the spool method is effective when the rewinding mechanism is provided.
The flange method has very high user operability  because the roll paper is held by flanges being inserted into the paper core from both sides, regardless of the size of the roll paper. However, only the following methods can be used to directly transmit driving force to the roll paper. In one method, a diameter of the flanges is set to be greater than a maximum outer diameter of the roll paper, and the driving force is transmitted to an outer diameter of the flanges. Alternatively, an edge of the paper serves as a base in a width direction when the paper is conveyed, and the driving force is transmitted to a flange on a side that is not the edge serving as the base. When the outer diameter of the flanges is increased, a machine size itself increases because of the increase in the diameter of the flange. Increasing the outer diameter of the flanges is disadvantageous in that one end of the roll paper is required to be lifted to set the roll paper. Layout is also restricted as a result of the flange diameter being increased. When the edge of the paper serves as the base in the width direction when the paper is conveyed, problems may occur in quality of conveyance, such as wrinkling, misalignment, and skewing, when a long paper is conveyed.
In Japanese Patent Application Laid-open No. 2003-276264 in which the flange method is used, a configuration is used in which a diameter of a flange member is made smaller than the maximum outer diameter of the roll paper  by a plurality of receiving rollers being provided. The receiving rollers hold the flange member such as to rotate freely. The roll paper is rewound by the receiving rollers being rotated in reverse. In the configuration, a rewinding mechanism is mounted on one of the receiving rollers. Stored force of a coil spring in the rewinding mechanism is actualized when the roll paper is transported. When the roll paper is rewound, the flange member is rotated in the rewinding direction by repulsive force of the spring. When rewinding speed is constant, an amount of slipping in the receiving rollers cannot be prevented from increasing, because a ratio of the outer diameter of the roll paper and a diameter of the receiving rollers or the diameter of the flange member changes as a result of decrease in an amount of remaining roll paper. Moreover, an amount of rewinding is limited by a fully wound state of the coil spring during transport. Therefore, the roll paper cannot be rewound over a long distance.